Photoaging is a term presently used to describe the changes in appearance and/or function of human skin as a result of repeated exposure to sunlight, and especially regarding wrinkles and other changes in the appearance of the skin.
Solar radiation reaching the earth's surface that effects and enables various animals, including humans, comprises ultraviolet (UV) (.lambda.&lt;400 nm), visible (400 nm&lt;.lambda.&lt;700 nm), and infrared (IR) (.lambda.&gt;700 nm). UV radiation is generally divided into UVA (320-400 nm), UVB (290-320 nm), and UVC (&lt;290 nm); UVC radiation is blocked from reaching the earth's surface by stratospheric ozone. The ultraviolet (UV) component of sunlight, particularly UVB, is generally believed to be the principal causative agent in photoaging.
The extent of UV exposure required to cause photoaging is not currently known, although the amount required to cause erythema (reddening, commonly seen as sunburn) in human skin is known and quantified empirically as the "minimal erythemal dose" ("MED") from a given UV source. UVB wavelengths of 290-300 nm are the most erythmogenic. The effectivenes of UV radiation in causing erythema decreases rapidly as the UV wavelength is increased beyond about 300 nm; wavelengths of 320 nm and 340 nm are, respectively, one hundred and one thousand times less potent at causing skin reddening than wavelengths of about 298 nm. Repeated exposure to sunlight at levels that cause erythema and tanning are, nevertheless, commonly associated with photoaging. Erythema from UVB is suggested to be a function of the total radiation exposure, not the intensity of the radiation exposure. According to Physiology, Biochemistry, and Molecular Biology of the Skin, 2nd Ed., ed. by L. A. Goldsmith (New York: Oxford Univ. Press, 1991), UVA is considered both melanogenic and erythemogenic and UVA exposure induces synthesis of a 32 kDa stress protein in human skin, as well as immediate erythema not apparent after UVB exposure.
Photoaging in human skin is characterized clinically by coarseness, wrinkles, mottled pigmentation, sallowness, laxity, eventually premalignant, and ultimately malignant neoplasms. Photoaging commonly occurs in skin that is habitually exposed to sunlight, such as the face, ears, bald areas of the scalp, neck, forearms, and hands.
Sunscreens are commonly used to prevent photoaging of skin areas that are exposed to sunlight. Sunscreens are topical preparations that contain ingredients that absorb, reflect, and/or scatter UV light. Some sunscreens are based on opaque particulate materials, among them zinc oxide, titanium oxide, clays, and ferric chloride. Because such preparations are visible and occlusive, many people consider these opaque formulations cosmetically unacceptable. Other sunscreens contain chemicals such a p-aminobenzoic acid (PABA), oxybenzone, dioxybenzone, ethylhexyl-methoxy cinnamate, octocrylene, octyl methoxycinnamate, and butylmethoxydibenzoylmethane that are transparent or translucent on the skin. While these types sunscreens may be more acceptable cosmetically, they are still relatively short-lived and susceptible to being removed by washing or perspiration.
As noted above, the generally accepted etiology of photodamage to skin involves an exposure to sunlight sufficient to cause erythema (sunburn or reddening; literally a flush upon the skin), and it is now known that sufficient UVB radiation does cause erythema. This philosophy dictates that present compositions and methods for inhibiting photoaging include the use compounds that block or absorb UVB, and that such compositions need be used only when there is sufficient likelihood that exposure to sunlight will result in erythema. More recent sunscreen compositions include combinations of compounds that block both UVA and UVB radiation.
It has been suggested that UV solar radiation induces reactive oxygen species (ROS) in the skin. Rieger, M. M. Cosmetics and Toiletries (1993) 108:43-56 reviews the topical application of known antioxidants to the skin for reducing the presence of ROS.
Retinoids have been used as therapy to improve the appearance of sun-damaged skin. U.S. Pat. No. 4,877,805 describes the treatment of photoaged skin. The patent indicates that there is little point in beginning the application of a retinoid to treat photodamage until the effects of aging begin to appear. Several studies have investigated improving the appearance of existing photodamaged skin with the use of all-trans retinoic acid. G. D. Weinstein et al., "Topical Trentinoin for Treatment of Photodamaged Skin," Arch. Dermatol., 127:659-665 (May 1991); J. S. Weiss et al., "Topical Tretinoin Improves Photoaged Skin," J Amer. Med. Assn., 259(4):527-532 (Jan. 22/29, 1988).
Matrix metalloproteinases (MMPs) are a family of enzymes that play a major role in physiological and pathological destruction of connective tissue, especially collagen. Various types of collagen and collagenases (types of MMPs) are known in this field, and a further description can be found in our copending U.S. patent application Ser. No. 08/588,771, filed Jan. 19, 1996, the disclosure of which is incorporated herein by reference in its entirety and for all purposes. Inhibitors of MMPs (i.e., direct inhibitors of the proteinase) and of molecular pathways (i.e., inhibitors of AP-1) that affect MMP expression are known in other fields and likewise are described in the aforementioned application number 588,771.
In summary, the state of the art considers that photodamage is caused primarily by UVB radiation, and that presently available sunscreens are sufficient to prevent photodamage. "Dr. Ceilley [current President fo the American Academy of Dermatology] believes that staying out of the sun and using sunscreen could have prevented many of the skin cancers that he treats in his practice, as well as the premature wrinkles that his patients are concerned about." Skin SAVVY, Amer. Acad. Dermat. supp. to USA Today, May 1997.